The present invention relates to variable displacement hydraulic pumps of the type having a rotating group and a tiltable cam member and swashplate for varying the displacement of the rotating group, and more particularly, to a swashplate centering and holddown mechanism for such pumps. Even more particularly, the present invention relates to such a mechanism in which the centering portion of the mechanism is adjustable.
Although the hydraulic pump, of the type with which the present invention may be utilized, may include various types of rotating groups, the invention is especially advantageous when used with a pump rotating group of the xe2x80x9cin-linexe2x80x9d axial piston type, i.e., one which includes a rotating cylinder barrel defining a plurality of cylinders, and a piston reciprocable within each cylinder, wherein the cylinders are parallel to each other and to the axis of rotation of an input shaft. Therefore, the present invention will be described in connection with such an in-line, axial piston pump.
Although the present invention may be used with various types of swashplate arrangements, it is greatly preferred that the invention be utilized in pumps of the xe2x80x9cswash-and-cradlexe2x80x9d type, as illustrated and described in U.S. Pat. No. 6,068,451, assigned to the assignee of the present invention and incorporated herein by reference. Therefore, the present invention will be illustrated and described in connection with an axial piston pump having a swash-and-cradle type of swashplate.
Changes in the displacement of an axial piston pump (by changing the tilt angle of the swashplate) may be accomplished either by an appropriate hydraulic servo mechanism, or by some sort of manual input. In the past, it was conventional practice that, if the displacement changes were to be accomplished by means of a hydraulic servo mechanism, the servo mechanism itself would include an appropriate centering device, i.e., a device which biases the servo, and indirectly, the swashplate also, toward its neutral (zero displacement) position. More recently, however, it has become more common to omit from the hydraulic servo mechanism the centering device (springs), and instead, locate within the pumping chamber a swashplate centering and holddown mechanism. Unlike the centering mechanism associated with the servo mechanism, the swashplate centering and holddown mechanism would accomplish both a centering function (zero displacement of the swashplate) and also a xe2x80x9cholddownxe2x80x9d function, by means of which the swashplate would be biased toward, and retained against, its adjacent cradle (bearing) surface.
Various centering and holddown devices have been designed by those skilled in the art, but, unfortunately, many of the prior art centering and holddown devices have been complicated and expensive, or have been difficult to assemble or adjust for neutral, or have involved some other operational disadvantage, such as imposing an undesirable side-load on the biasing springs.
An example of such a prior art centering and holddown mechanism, and one which is still in widespread commercial use, is shown in U.S. Pat. No. 4,584,926. In the device of the ""926 patent, the adjustment of the centering mechanism is accomplished, at the time of pump assembly, by rotatably adjusting a relatively large, externally-threaded plate member which is received within a set of internal threads defined by the pump housing. The internal and external threads defined by the housing and the plate, respectively, add substantially to the overall machining and manufacturing cost of the pump. In addition, it has been observed that the rotatable plate, being held in place relative to the pump housing only by means of the threaded connection, can serve as a source of resonant noise and, under the right conditions, can actually amplify whatever noise is generated.
It has also been observed in connection with the mechanism of the ""926 patent that, once the device is adjusted to achieve absolute neutral (zero displacement), the spring seat may no longer be perfectly perpendicular to the axis of rotation of the pump. Those skilled in the art will understand that the lack of perfect perpendicularity of the spring seat can impose a side load on the biasing springs of the mechanism which can reduce the life of the springs. Those skilled in the art will understand also that the performance of the mechanism of the ""926 patent is very much dependent upon maintaining extremely close tolerances between the various points of mechanical connection within the mechanism, as well as between the mechanism and the pump housing.
Accordingly, it is an object of the present invention to provide an improved swashplate centering and holddown mechanism of the type in which adjusting the mechanism for absolute neutral (zero pump displacement) does not introduce any other disadvantages in the pump operation, such as vibration and noise, or a side load imposed on the biasing springs.
It is another object of the present invention to provide such an improved swashplate centering and holddown mechanism, which achieves the above-stated object, and in which it is possible to utilize relatively looser manufacturing and assembly tolerances among the various parts without losing the ability to consistently achieve substantially absolute neutral (zero pump displacement).
The above and other objects of the invention are accomplished by the provision of an improved swashplate centering and holddown mechanism for a variable displacement axial piston unit comprising a housing defining a chamber, and an axis of rotation, a cylinder barrel disposed for rotation about the axis of rotation, the cylinder barrel defining a plurality of bores and having a plurality of pistons axially moveable therein. The unit includes a cam member tiltable about a transverse axis, perpendicular to the axis of rotation, and having a swashplate operably associated with each of the pistons to cause reciprocal movement thereof in response to rotation of the cylinder barrel, when the cam member is displaced from a neutral position, in which the swashplate is perpendicular to the axis of rotation, to a displaced position. The swashplate centering and holddown mechanism biases the cam member axially toward a cradle surface and pivotably toward the neutral position.
The improved mechanism is characterized by a pair of arms, each of the arms defining a pivot location, at one axial end thereof, fixed relative to the housing on one side of the axis of rotation, and a swashplate-engaging portion, at the opposite axial end thereof, engaging the swashplate, on the other side of the axis of rotation, when the swashplate is in the neutral position. A connector is operably associated with the arms, whereby the arms are able to pivot about the pivot locations in a generally scissors-type movement. A biasing means biases the swashplate-engaging portions of the arms towards the swashplate, whereby, in the absence of an input to tilt the cam member, the swashplate is in engagement with both of the swashplate-engaging portions and is in the neutral position.